Field of the Invention
The present invention relates to an image reading device to be used in an image forming apparatus, for example, the image reading device being configured to read an image on a sheet of document conveyed on a scanner glass platen in a sub-scanning direction, a method for the image reading device to detect a foreign body on the scanner glass platen, and a recording medium.
Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
Image forming apparatuses are commonly provided with an image reading device with an on-board automatic document feeder. Since a sheet of document is conveyed in a sub-scanning direction on a scanner glass platen of such an image forming apparatus, a scanned image may contain a linear noise area, which stretches in a document conveying direction, due to a foreign body such as lint, dust, or dirt on the scanner glass platen.
To remove such a noise area, the following methods have been already proposed and known: a method of detecting a foreign body on a scanner glass platen and a method of detecting a noise area due to a foreign body from a scanned image.
For example, Japanese Unexamined Patent Application Publication No. 2000-287039 discloses a technique of identifying the position of a foreign body and replacing the affected pixels with white pixels.
For another example, Japanese Unexamined Patent Application Publication No. 2002-185767 discloses a technique of counting up all lines in a sub-scanning direction and detecting noise when the level is equal to or greater than its threshold value.
According to the publications, these techniques are provided with one photo-sensor, such as a CCD image sensor, disposed in a main scanning direction to read a document image. This configuration with one photo-sensor allows identifying the source of noise as either a sheet of document or a foreign body such as lint on the scanner glass platen, using one single scanned image. These techniques, therefore, do not cover detecting a foreign body with a high degree of accuracy; in other words, these are hardly capable of correctly identifying the source of a liner noise area stretching in a direction, in which a sheet of document is conveyed, as either a sheet of document or a foreign body on the scanner glass platen.
To achieve accurate detection, Japanese Unexamined Patent Application Publication No. 2002-271631 discloses a technique of scanning images by a 4-CCD image sensor with a gray (Gr), red (R), green (G), and blue (B) channels and comparing these images using the interval between the lines for these channels, which are arranged in a document conveying direction.
However, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-271631 is based on the assumption that the size of a foreign body is equal to or smaller than the interval between the lines of the CCD image sensor; that is, it is hardly capable of detecting a foreign body whose size is larger than the interval between the lines. Furthermore, the technique is not capable of detecting a foreign body hiding all the Gr and RGB channels.
As described above, a foreign body such as lint, dust, or dirt on a scanner glass platen causes a linear noise area in a scanned image; the imaging performance of an image reading device with an on-board automatic document feeder is thus affected negatively. Such an image reading device does not cover detecting a foreign body with a high degree of accuracy; in other words, it is hardly capable of correctly identifying the source of noise as either a sheet of document or a foreign body on the scanner glass platen.